1. Field of the Invention
The present invention relates to a digital printer and an image data conversion method for the digital printer wherein image data obtained from a photographic image is processed to be digital image data for reproducing the image as a hard copy.
2. Background Arts
In a conventional digital printer, an image signal is obtained from a reflective original such as a photo-print or a transparent original such as a frame on a photographic film, and the image signal is processed to obtain image control values. Based on the image control values, an image is reproduced and recorded on a photographic paper or another recording material. For example, JPA 2-157758 discloses determining a reference density point for each color component based on a highlight density and a shadow density in the original, and setting up a gradation conversion curve to assign these reference density values to given signal levels.
JPA 6-242521 discloses dividing a scene into several areas to detect a maximum value and a minimum value of each color from each area, determining a maximum reference value and a minimum reference value from these maximum and minimum values. The maximum reference values of three colors, i.e. red (R), green (G) and blue (B), are reproduced as white, whereas the minimum reference values of three colors as black.
JPA 6-178113 discloses producing a histogram from image data, accumulating histograms of a plurality of images according to the type of recording media, and determining gradation curves based on the image data and the accumulated histogram data, to produce a conversion table for the image reproduction. This publication also discloses modifying the histogram to eliminate lopsided frequencies in some density levels, and selecting those images relating to one another among from a series of images, to utilize as image representative values for reproduction conversion curves.
JPA 2-184837 discloses obtaining characteristic curves of negative films under single-color exposures, storing these curves, and converting improperly exposed originals such as under-exposed and over-exposed negatives into single-color exposure densities, to reproduce the images after correcting the non-linearity of the characteristic curves of the improperly exposed originals.
Using previously stored reference gradation curves for setting up a gradation of a digital image is widely known, for example, from JPA 60-3778, JPA 60-216350, JPA 62-111569, JPA 62-111571 and JPA 63-42575. JPA 60-37878 discloses previously storing a plurality of standard gradation curves, and selecting among from those standard gradation curves a most proximate to a gradation curve that is produced from an original. JPA 60-216350, JPA 62-11569 and JPA 62-111571 disclose correcting a stored reference gradation conversion curve according to data of specific points. JPA 63-42575 discloses using a third gradation conversion curve that is obtained by composing a standard gradation conversion curve with a gradation conversion curve determined based on an original.
In almost all conventional digital image recording methods, highlight and shadow of the original are controlled to be reproduced as highlight and shadow in the reproduced image, as above described JPA 2-157758. This method is disadvantageous when the highlight and shadow of the original are not white and black. In a case where a scene is divided into a plurality of regions to determine a maximum reference value and a minimum reference value from maximum values and minimum values of respective colors in the respective regions, like JPA 6-242521, dividing the scene can result in wrong reference values. To determine single-light exposure characteristic curves for each film type like in JPA 2-184837 is not useful in practice. The method as disclosed in JPA 6-178113, wherein a histogram is made from image data and the histogram is accumulated in a memory provided for each type of recording media, is useful for those recording media like reversal films where exposure amounts on photography are substantially uniform and most images are properly exposed. However, for those recording media like negative films where color balance, gradation and other characteristic values vary depending upon exposure amounts on photography, the histogram varies according to the exposure amount even for the same photographic subject, so that it is difficult to exactly obtain necessary film properties from simple accumulation of the histograms.
FIGS. 19A to 19C are density histograms for red (R-density) of images of the same subject recorded on a negative film, but respectively under-exposed, properly exposed and over-exposed ones. As seen from these diagrams, even if the images are photographed from the same subject on the same negative film, their histograms vary depending upon exposure conditions. Concerning originals on negative film, accumulation of their histograms does not result in a histogram for an average subject, but vainly equalizes and flattens the histograms.
Color balance of the negative film also varies according to photographic exposure amount, film type, the light source for photography, color fading of the photographic image. Especially because each type of film is designed to provide an optimum color balance in combination with a particular recording material, a different combination can result in improper color balance and thus unsatisfactory reproduction quality. Moreover, according to the conventional color balance setting up method, reproduction conditions are determined for each image based on image data of each image, there can be color variance between the reproduced images.